This Core is intended to: 1. permit rapid and safe exchange of germline mutant mice between La Jolla and[unreadable] Osaka as frozen embryos, circumventing the delay imposed by quarantine; 2. secure the valuable strains[unreadable] created using ENU; 3. provide for the rescue of strains with low fertility and accelerate the fixation of[unreadable] mutations that can only be fixed by inbreeding, given that the MCMV screen is lethal to homozygotes; 4.[unreadable] provide for the distribution of mutant strains to the scientific community at large by expanding and shipping[unreadable] homozygous stocks to the MMRRC. This Core will be run using techniques that we have established and[unreadable] have used routinely, and requires no expenditure for equipment. It will be mirrored by a comparable Core in[unreadable] Osaka, Japan, which will focus primarily upon items 1, 2, and 4 (above) to the exclusion of IVF. Neither[unreadable] Core can function in the absence of the other, since embryos frozen in one Core must be re-derived in the[unreadable] other, and vice versa. Hence they should each be viewed as separate but essential Cores for two of the[unreadable] participating groups (in La Jolla and Osaka).